The Hermann grid illusion is an optical illusion, a visual perception that differs from
objective reality. The illusion is characterized by "ghostlike" grey blobs perceived at the intersections of a
white (or light-colored) grid on a black background. The grey blobs disappear when looking directly at an
intersection. The scintillating grid illusion, discovered by E. Lingelbach in 1994, is variation of the
Hermann grid illusion. It is constructed by superimposing white discs on the intersections of orthogonal gray
bars on a black background. Dark dots seem to appear and disappear rapidly at random intersections, hence
the label “scintillating”. When a person keeps his or her eyes directly on a single intersection, the dark dot
does not appear.
You can find information about the Hermann Grid illusion in my Lecture notes for Class 5 – Neurons and
Perception – Part 1.
Purpose: To familiarize yourself with the Hermann grid illusion, and those factors that affect it.
Task: This assignment will assess what properties of the stimulus affect both of these illusions. For this
assignment you will be using the Hermann Grid (curving) illusion from Michael Bach’s website:
http://www.michaelbach.de/ot/lum-herGridCurved/index.html
Optical illusions & visual phenomena and explanations (where possible):
Optical Illusions & Visual Phenomena (michaelbach.de)
Before you begin, disable the switching of the illusion from boxes to curved lines by removing the check on
the Auto-Run button.
Using your knowledge of how the retina is structured, along with the organization of receptive fields,
describe and explain your perceptions of the following questions:
Please write legibly!
1) How is your perception of the Hermann grid affected by changes in width of the grid lines (change the
street lines wheel to make bigger or smaller)? Why?
2) The illusion is the strongest when the grid lines are mid-sized. Why?
3) What happens to the illusion when you decrease the contrast between the lines and the boxes? (i.e. gray
lines, black boxes using the colors box to change the line and box colors
4) Are there any colour combinations that make the illusion weaker? Why?
5) Why do the blobs not appear at the point of fixation, and only in the periphery?
